Why have the subgenres of stoner metal, sludge metal, and doom metal so popular recently?

Anyone have any particular reasons why? While these genres are very metal I know people who are into these genres that I wouldn't call metalheads, but they're into stoner, sludge, and doom. Also when I say I wouldn't call them metalheads I mean that if you were to ask them who Chuck Schuldiner is or who Emperor are they wouldn't be able to give you an answer.

vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Edit: I like all of these genres as much as I like death, thrash, power, or whatever, but I'm just wondering why these genres are more accessible to outside of the metalcrowd (whom I have nothing against!). I'm not saying death is better then sludge or vice versa. I want answers as to why sludge is more popular then death. Why? Because I'm curious. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^